A gas is a state of matter, the others being solids and liquids. Gases are distinguished from the other states by the amount of energy the constituent molecules have. With gases the molecules making up the gas have virtually completely free movement within their container. If the volume is increased then the gas expands to fill the new volume. As this happens the pressure that the gas is exerting on the walls of the chamber decreases as there are fewer molecules colliding with a given area of the containers walls. A gas is distinguished from a vapor in that a gas is above the critical point at which the liquid boils.

There are several laws and equations that describe the behavior of gases under specified conditions, collectively they are known as the gas laws.Boyle's law describes the behavior of a gas when the temperature is kept constant...